A personal communications system (PCS), like many other wireless communication systems, is organized around the cellular concept. The cellular concept involves dividing a customer-site, which is an area over which radio coverage is desired, into smaller areas called cells. Each cell contains a fixed base station near its centre that is networked with all the other base stations on the site. Each base station receives messages from and transmits messages to all handsets being used within its cell. The cellular concept also involves changing the base station that a handset relies upon to communicate with the network, as the handset is moved from one cell to another within the customer site, in a process referred to as a hand-off.
The efficient deployment of a system that is based on the cellular concept, involves providing radio coverage over all parts of a customer site at the lowest possible cost. This task, in turn, requires knowledge of the exact points in the customer site at which each cell's boundaries occur. A cell boundary occurs at the furthest points from a base station where signals to and from its base station are still of customer approved quality. The goal of deployment, is to set the cell-boundaries so that the minimum number of cells are planned for the area over which radio coverage is required, and so that all of the area is covered.
Note that the process of accurately ascertaining cell boundaries at the deployment stage is not to be confused with the process of efficiently effecting hand-offs in a live network. The hand-off process involves continually reselecting base stations in a noisy environment in order to maintain a communication channel as a wireless handset is moved by its user from one cell to another. Cell boundary determination involves determining the shape and location of a cell within which a handset can effectively communicate with a base station in a base station deployment. Accordingly, different sets of requirements, obstacles, and resources are associated with the two processes.
It is common in the industry to ascertain the location of cell boundaries by using intuitive or predictive techniques that are collectively classified as passive deployment procedures. Lucent Technologies' WISE tool embodies an example of such a procedure. These passive procedures, though often relatively cheap to conduct, often yield inadequate results for many reasons. One such reason is that the nature of wireless propagation is not well understood by many deployment engineers. Though radio signals may radiate from a base station uniformly in all directions, they can be attenuated by obstructions in their paths (for example, walls, metalwork, concrete, hills etc.) in a process referred to as long-term fading. Modelling long-term fading requires considerable skill, experience and a tremendous amount of site specific information; when not done by highly skilled engineers, or when not done without such information, installation errors result. Another reason why passive deployment procedures yield inadequate results is that even when they are carried out by a skilled deployment engineer, difficulties are often encountered because the radio opacities of many elements (e.g. floors, walls, ceilings) cannot be determined by visual inspection. In such cases, accurate modelling using passive techniques degenerates into a more speculative guessing technique.
The foregoing problems can be avoided using radio deployment tools (RDTs) that enable installers to carry out what are known in the art as active deployment procedures. Active deployment procedures, when considered in contradistinction to passive deployment techniques, involve determining cell-boundaries by analysing radio signals propagating from base stations. An RDT used to carry out active deployment procedures consists of a wireless handset that the deployment engineer uses to generate test signals and which will be referred to herein as a radio deployment tool wireless handset (RDTWH), and a base station that is used to measure the received signal strength intensity (RSSI) of the test signals sent by the RDTWH, and which will be referred to herein as a radio deployment tool base station (RDTBS).
One of the methods by which a deployment engineer uses an RDT to determine a cell boundary is as follows. The deployment engineer deploys an RDTBS in a location that provides coverage over an extremity of the site. The deployment engineer then walks radially away from the RDTBS in a fixed direction with an RDTWH in hand that continuously transmits a test signal back to the RDTBS. The RDTBS monitors the RSSI-level of the test signal to ensure it remains above some predetermined threshold value, RSSI.sub.0. The deployment engineer keeps walking away from the RDTBS until the RSSI of the test signal falls below RSSI.sub.0 at which point the RDTBS indicates to the RDTWH that a point on the cell boundary has been reached. When the RDTWH, in turn, notifies the deployment engineer of this occurrence through a noise, a message on its display or some other input/output (I/O) means, the deployment engineer notes his or her current location with respect to the RDTBS, and deems that location to be a boundary-point of the cell currently being measured.
It is to be noted that RSSI.sub.0 is not set equal to the minimum power level at which communication between a handset and a base station is of an acceptable quality, this minimum power level being referred to hereinafter as RSSI.sub.min. This is because a fading margin is required to allow for non-linear and unpredictable fading effects which may occur during the actual use of a handset in a non-deployment environment but which may not be detectable in a deployment environment. As such, RSSI.sub.0 is set to equal RSSI.sub.min +M.sub.f, where M.sub.f is the required fading margin. In making provision for a fading margin, it is guaranteed that during actual use of a handset at any point in a cell, a degradation in the RSSI-level of up to M.sub.f dB due to non-linear and unpredictable fading may be tolerated without the RSSI-level dropping below RSSI.sub.min.
It also is to be noted that in order for the continually moving deployment engineer to receive cell-boundary information in a timely manner, the time taken for (i) the RDTBS to determine that the RSSI is at the threshold value, (ii) the RDTBS to inform the RDTWH that a threshold has been reached, and (iii) the RDTWH to report this information to the deployment engineer, should ideally be less than one second.
The foregoing process is repeated until a plurality of boundary-points that collectively constitute a cell-boundary are defined.
When the deployment engineer has finished marking out a cell's boundary using the RSSI readings for the test signals displayed on the RDTWH, he or she then locates a new RDTBS in a cell adjacent to the cell whose boundary was just defined, such that it covers the points at which the RSSI-levels of the previous cell's RDTBS signals start to drop below an acceptable threshold. The process described above is then repeated for each new cell in succession, until coverage for the entire customer-site has been established.
Thus, by a series of such RSSI measurements, all the cell boundaries in a site can be accurately and quickly determined, and ultimately, an acceptable positioning of base stations can be achieved regardless of any sources of non-linear and unpredictable fading effects that are particular to a given customer-site.
Unfortunately, the performance of existing RDTs becomes inadequate when installing higher bit-rate PCS networks supporting protocols such as the Digital European Cordless Telecommunications (DECT) protocol. More specifically, existing RDTs are incapable of accounting for the effects of temporal dispersion and short-term fading, even though these effects need to be factored into the determination of cell boundaries when deploying high bit-rate PCS networks.
Temporal dispersion occurs when a symbol is transmitted through a time-dispersive medium such that the symbol is smeared by the time it arrives at the base station. Short-term fading occurs when a receiver receives many echoes of one transmitted signal at staggered intervals of time. Temporal dispersion and short-term fading need to be accounted for because they can cause bit-errors to occur in a non-deployment environment with a frequency which may make the signal quality unacceptable.
Prior-art RDTs are incapable of accounting for dispersion however, because dispersion affects signals by disrupting the timing of their propagation, not by causing an attenuation in signal power. Moreover, prior art RDTs also do not account for short-term fading because this effect causes RSSI-variations which are short in duration and which are typically averaged out over a longer period of time. This leaves prior art RDTs, which can only differentiate test signals by their RSSI-levels, and which do not attempt to control the RSSI-levels at which test-signals are received, with no way of delineating strong signals that are unaffected by dispersion and short-term fading from strong signals that are affected by dispersion and short-term fading.
This is an especially serious shortcoming, because there is no real way for the deployment engineer to account for these effects using passive deployment procedures. In the radio-static deployment environment, dispersion is highly dependent on the radio propagation medium, and with the exception of obviously reflective environments such as train stations with metal walls, the ability of the deployment engineer to ascertain which environments will be effected by high-levels of dispersion and short-term fading without a tool that can actively measure these effects, is impractical.
This failure of prior art RDTs to account for dispersion and short-term fading means that they can incorrectly indicate to the deployment engineer that no cell boundary should be defined even at a point in the site where there is an inadequate fading margin.
Some packet-based communication protocols, such as the Digital European Cordless Telecommunications (DECT) protocol for example, have packet formats that include both cyclic redundancy check (CRC) fields and check sum (CS) fields. These fields are used to perform an integrity check on the various sub-fields of the entire packet, and are only capable of indicating whether or not a packet is error-free. It is impossible to obtain high-resolution bit error rate (BER) measurements using the CRC or CS fields. This is largely because, even in high bit-rate protocols such as the Digital European Cordless Telecommunications (DECT) protocol, a packet can only be sent by the RDTWH to the RDTBS once every 10 ms, and because the CRC and CS fields only indicate that either no bit errors have occurred, or one or more bit errors have occurred. This means that any BER-levels determined using CRC or CS fields can only be accurate to approximately 1%. This is a substantially inferior resolution to the BER resolution of 0.1%, which is the BER-level below which errors in a signal are imperceivable to the typical PCS user.